Here is the best explanation I've seen:
My understanding is: the way the law is written, whenever the FCC releases draft rules they have to open up a public comment period, which takes weeks or months.
So if they released it before the vote, they'd need to open up another comment period. Then if they made changes and released it again, another comment period - an infinite loop. Given the law as it stands, there's always one last phase where they have to go heads down and vote on a rule.
Yes, that means that nobody outside a tiny number of people at the FCC has read the final version. And yes, it's crazy the law is written this way.
That said, there are several people in the community of groups that have been working on this who were in constant conversation with the people drafting these rules and have a good idea of what's in them. That includes Free Press, as well as experts like Barbara van Schewick. So once we got close to the vote, and had good intel that the rules were pretty strong, it would have been crazy to ask them to be released. It would've given Comcast and friends another 8 weeks or maybe more to regroup and figure out a way to block the rules. (And we'd still not be in a position to know exactly what the final rules were until after the vote, due to the problem above.)
Also, re: the 322 pages of rules, I've heard the actual rules themselves are just a few pages. Things like: "no blocking", "no throttling", etc. The rest is context, legal justification of the rules, stuff like that.
All the details matter of course, but it's not really the case that there are 322 pages of mystery rules. Another really sleazy thing that some critics of the FCC (including Commissioner Pai himself) have done here is they've taken aspects the way the FCC has always worked (and they know this) and then pretended to be absolutely shocked that it's acting this way now.
One example of this was the fake outrage at President Obama saying what he thought the FCC should do (many presidents have done this, including Bush, Reagan, Clinton, and Nixon). Another example is this snafu about transparency, or about "322 pages of rules". It's not that the FCC shouldn't be more transparent, but it's sleazy of these guys, who know the process well, to pretend that it doesn't always work this way.